Anaesthetic vaporizers are well known in the art and a large number of different methods of application are described in the literature. For a better understanding of the known and used vaporizers, reference is made to Anaesthetic Equipment, C. S. Ward, publisher Bailliere Tindall, 2nd edition, 1987, pp. 78-103, and to Anesthesia Vaporizers by J. B. Eisenkraft in Anesthesia Equipment, principles and applications, authors Jan Ehrenwerth, James B. Eisenkraft, publisher Mosby 1993, pp. 57-58.
The earlier described vaporizers are based on the principle of storing the liquid anaesthetic in a container. A breathing gas is introduced into this container and caused to pass over the surface of the liquid or to percolate therethrough.
Some of the anaesthetic is vaporized during passage of the breathing gas and therewith delivered to the patient together with the gas. This method, however, is encumbered with a large number of problems.
1. As the anaesthetic is vaporized, energy is removed from the gasified liquid, therewith cooling the gas. This cooling of the gas can result in a change in the vapor pressure across the liquid, and therewith in changes in the amount of anaesthetic that is entrained with the breathing gas. PA1 In an endeavour to overcome this problem, additional heat is supplied in a number of constructions or, in the case of temperature-sensitive systems, the amount of breathing gas that passes across the liquid surface is varied and different gas flows are then combined so as to obtain a constant anaesthetic concentration in the breathing gas. PA1 2. Vaporization of the anaesthetic is contingent on the rate of flow of the breathing gas. It has been endeavored to compensate for this dependency, by including different intricate flow-dependent valves and gas mixing systems in the vaporizer. This flow dependency can cause problems, particularly in the case of low fresh-gas inlet flows used in so-called low flow systems. PA1 3. Different anesthetics have different vaporization characteristics and must be applied in given concentrations for optimal anaesthesia. Attempts have been made to compensate for this, by designing individual vaporizers for solely one anaesthetic. One serious drawback in this regard is that erroneous filling of a given vaporizer with an anaesthetic for which the vaporizer is not intended can have catastrophic effects. The requirement of several different vaporizers mounted together on an anaesthesia machine also introduces the risk of all vaporizers being in operation simultaneously, with the danger of overdosing the anaesthetic. PA1 4. Anesthetics have different vaporization characteristics in different gas mixtures. This can result in administering to the patient an anaesthetic concentration that is different to the concentration for which the vaporizer is set, due to the composition of the gas mixture. PA1 5. A number of systems are based on the principle of submerging a wick in the anaesthetic. The anaesthetic is sucked up by the wick and vaporized on its surface. One drawback with this system, however, is that the suction rate is dependent on the height and temperature of the liquid surface, whereby the vaporizer must include a compensating system.
DE-A 4 105 163 teaches an anaesthetic vaporizing system in which a porous body is saturated with anaesthetic and through which the anaesthetizing gases pass.
One drawback with this system is that the total quantity of anaesthetic to be used is limited to the absorbency of the porous body. Another drawback is that evaporation of the anaesthetic in the by-passing gas will vary with time, due to lowering of the temperature of the body among other things (this lowering of temperature being caused by gas evaporation). The system must therefore include a separate temperature control circuit in order for the system to function satisfactorily. The system includes no pump or active supply means for delivering anaesthetic gas to the absorption-desorption material.
U.S. Pat. No. 4,0155,599 discloses that the absorbent maintains the anaesthetic in a two-dimensional state (it is not clear from the document what is meant by this). The anaesthetic is kept in a liquid state by means of a wick on the other hand. This system also includes a charged bed of absorbent through which gases pass. One drawback with this known system is that the temperature must be controlled and that different evaporation-absorption rates are obtained with mutually different anaesthetic gases.
U.S. Pat. No. 3,540,445 describes a vaporizer in which fibrous wicks are replaced with porous synthetic plastic materials that absorb the anaesthetic from a container through the medium of capillary forces. Although the container can be replenished, or topped-up, the amount of anaesthetic delivered to the by-passing breathing gas is determined mainly by evaporation from the porous plastic rods and the capillary forces within the rods (when the anaesthetic is maintained at a constant level in the container), wherewith the system becomes temperature-dependent and also contingent on the anaesthetic to be vaporized.
GB 2 255 912 describes a system that includes porous rods through which part of the gas passes while another part of the gas passes past the rods. The rods are supplied with gaseous anaesthetic, by submerging the rods in liquid anaesthetic. The system includes a level regulator which functions to control the level of the liquid anaesthetic in relation to the rods. It is necessary to control the rods and the temperature of the anaesthetic and the gas, in order to obtain a stable concentration of anaesthetic in the gas.
GB 2 279 015 describes a device in which the liquid to be vaporized is exposed to the breathing gas, partly through the medium of pores and partly through the medium of the free liquid surface. Consequently, the device also includes temperature control means. The device has no facility that enables liquid volumes to be regulated.